The present invention relates generally to generators that provide a power output, and in particular to power generators that supply a modulated output waveform.
Generators that provide a modulated output are limited in several respects by the waveform sensing/control techniques that are used for controlling the output. Typically, conventional generators employ a control method that tracks the positive-going slope of a sense signal that is representative of the output waveform. The rising edge of the sense signal is tracked by a charge-dump-hold circuit (CDH circuit). The CDH circuit continues tracking the waveform until, after a predetermined amount of time to account for plant settling time, the value of the sense signal is measured. A hold circuit is activated simultaneously with the disabling of the output modulation, providing an effective continuous DC output regardless of the modulation parameters. Output characteristics, such as delivered power, are computed by multiplying the measured parameter by the pulsed output duty cycle. For example, for a 50 percent duty cycle, with a peak power measurement of 1000 watts, the calculated delivered output power would be 500 watts.
Limitations of the described sensing/control technique include the need for pre-measuring the settling time of the plant, the CDH circuit response limits the bandwidth of the system, the technique cannot track both the positive and negative edges of the output waveform, and arbitrary amplitude output waveforms are not controllable. The settling time is generally obtained based upon lab experiments, and stored for use during system operation. The settling time of the plant will vary for many systems based on the load impedance that is being driven. By using a settling time based on a single load impedance, the accuracy of the calculated output parameters may vary depending on the load impedance, limiting use of the control technique to matched systems or systems that can withstand high levels of inaccuracy in the computation of the output characteristics.
The restriction on system bandwidth imposed by the CDH circuit response time may be the limiting factor in determining the minimum pulse width of the output waveform. Having the measurement system (i.e. the CDH circuit) limit the speed of the generator system is opposite to normal engineering practice. An inability to track both the positive and negative edges limits the control technique to controlling output waveforms in which a pulse returns to zero during the low cycle of the modulating waveform. Controlling arbitrary amplitude modulation waveforms without the ability to track both edges is generally unattainable. Therefore, conventional generator control techniques are typically limited to controlling pulsed output waveforms unless the generator system can tolerate inaccurate generation and measurement of output characteristics.
While conventional generator control techniques can be used to provide a modulated output waveform under limited operating conditions, those techniques have not proven capable of providing generators that are not limited by the sensor/measurement circuit.
A method and system of controlling an output of a generator is provided. The output provides an output signal having a settling time to a load having an impedance. The settling time of the output signal is determined. The output signal is amplitude modulated with a modulation waveform. A sense signal is generated that is representative of the modulated output signal. The sense signal is sampled at a sampling time based upon the settling time of the output signal. A digital representation of the sampled sense signal is generated. The amplitude modulation of the output is controlled based upon the digital representation of the sampled sense signal.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limited the scope of the invention.